22.2.1 Enabling the FW-H Acoustics Model

To enable the FW-H acoustics model, select Ffowcs-Williams & Hawkings in the Acoustics Model dialog box (Figure  22.2.1).

Models Acoustics Edit...

When you select Ffowcs-Williams & Hawkings, the dialog box will expand to show the relevant fields for user inputs.

Figure 22.2.1: The Acoustics Model Dialog Box

Setting Model Constants

Under Model Constants in the Acoustics Model dialog box, specify the relevant acoustic parameters and constants used by the model.

Far-Field Density   (for example, $\rho_0$ in this equation in the separate Theory Guide) is the far-field fluid density.

Far-Field Sound Speed   (for example, $a_0$ in this equation ) is the sound speed in the far field (= $\sqrt{\gamma R T_0}$).

Reference Acoustic Pressure   (for example, $p_{\rm ref}$ in Equation  29.11-11) is used to calculate the sound pressure level in dB (see Section  29.11.4). The default reference acoustic pressure is $2 \times 10^{-5}$ Pa.

Number of Time Steps Per Revolution   is available only for steady-state cases that have a single rotating reference frame. Here you will specify the number of equivalent time steps that it will take for the rotating zone to complete one revolution.

Number of Revolutions   is available only for steady-state cases that have a single rotating reference frame. Here you will specify the number of revolutions that will be simulated in the model.

Source Correlation Length   is required when sound is to be computed using a 2D flow result. The FW-H integrals will be evaluated over this length in the depth-wise direction using the identical source data (see Figure  22.2.2).

The default values are appropriate for sound propagating in air at atmospheric pressure and temperature.

Computing Sound "on the Fly''

The FW-H acoustics model in ANSYS FLUENT allows you to perform simultaneous calculation of the sound pressure signals at the prescribed receivers without having to write the source data to files, which can save a significant amount of disk space on your machine. To enable this "on-the-fly'' calculation of sound, enable the Compute Acoustic Signals Simultaneously option in the Acoustics Model dialog box.

Because the noise computation takes a negligible percentage of memory and computational time compared to a transient flow calculation, this option can be used by itself or along with the process of source data file export and sound calculation. For the latter, computing signals "on the fly'' allows you to see when the signals have become statistically steady so you can know when to stop the simulation.

When the Compute Acoustic Signals Simultaneously option is enabled, the ANSYS FLUENT console window will print a message at the end of each time step indicating that the sound pressure signals have been computed (e.g., Computing sound signals at x receiver locations ..., where x is the number of receivers you specified). Enabling this option instructs ANSYS FLUENT to compute sound pressure signals at the end of each time step, which will slightly increase the computation time.

Note that this option is available only when the FW-H acoustics model has been enabled. See below for details about exporting source data without enabling the FW-H model.

Writing Source Data Files

Although the "on-the-fly'' capability is a convenient feature, you will want to save the source data as well, because the acquisition of source data during a transient flow-field calculation is the most time-consuming part of acoustics computations, and you most likely will not want to discard it. By saving the source data, you can always reuse it to compute the sound pressure signals at new or additional receiver locations.

To save the source data to files, enable either the Export Acoustic Source Data in ASD Format or the Export Acoustic Source Data in CGNS Format option, or both. After you have made your selection, the relevant source data at all face elements of the selected source surfaces will be written into the files you specify. The source data vary depending on the solver option you have chosen and whether the source surface is a wall or not. Table  22.2.1 shows the flow variables saved as the source data.

Figure 22.2.2: The Acoustics Model Dialog Box for a 2D Steady-State Case with a Single Rotating Reference Frame

Table 22.2.1: Source Data Saved in Source Data Files

Solver Option	Source Surface	Source Data
incompressible	walls	$p$
incompressible	permeable surfaces	$p, u, v, w$
compressible	walls	$p$
compressible	permeable surfaces	$\rho, p, u, v, w$

See Section  22.2.2 for details on how to specify parameters for exporting source data.

Exporting Source Data Without Enabling the FW-H Model: Using the ANSYS FLUENT ASD Format

You can export sound source data for use with SYSNOISE without having to enable the Ffowcs Williams and Hawkings (FW-H) model. You will still need to specify source surfaces (see Section  22.2.2), as .index and .asd files are required by SYSNOISE. In addition, you can choose fluid zones as emission sources if you want to export quadrupole sources. To enable the selection of fluid zones as sources, use the

define $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ export-volumetric-sources?

text command and change the selection to yes.

SYSNOISE also requires centroid data for source zones that are being exported.

For fan noise calculations, once you have specified the source zones in the Acoustic Sources dialog box and you have selected Export Acoustic Source Data in ASD Format from the Acoustics Model dialog box, you can export geometry in cylindrical coordinates by using the

define $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ cylindrical-export?

text command and changing the selection to yes. By default, ANSYS FLUENT exports source zones for SYSNOISE in Cartesian coordinates.

You can then export the centroid data to a data file using the following text command:

define $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ write-centroid-info

Since you will not be using the FW-H model to compute signals, you will not need to specify any acoustic model parameters or receiver locations. Also, you will not be able to enable the Compute Acoustic Signals Simultaneously option in the Acoustics Model dialog box, and Acoustic Signals... will not be available in the Run Calculation task page.

Exporting Source Data Without Enabling the FW-H Model: Using the CGNS Format

The sound source data for non-permeable surfaces can be exported in the CGNS file format (for Virtual Lab) without having to enable the Ffowcs Williams and Hawkings (FW-H) model. Enable the Export Acoustic Source Data in CGNS Format option in the Acoustics Model dialog box (Figure  22.2.3). Specify the source surfaces in the Acoustics Sources dialog box (see Section  22.2.2) where, by default, the Number of Time Steps per File is set to 1.

Figure 22.2.3: The Acoustics Model Dialog Box for Exporting in CGNS Format

Virtual Lab requires a mesh data file (named <prefix>_mesh.cgns) and a solution data file (named <prefix>_<timestep>.cgns). The string <prefix> is a generic name, which you will specify in the Source Data Root File Name in the Acoustics Sources dialog box. There is one single solution data file (.cgns) per time level exported, which contains the static pressure at the wall-face centroid location. The .cgns files will be stored in a directory, which you specify (named <directory_name>/<prefix>) in the Source Data Root File Name.

In addition, you can export quadrupole sources data by choosing fluid zones as emission sources. To enable the selection of fluid zones as sources, use the text command:

define $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ export-volumetric-sources-cgns?

When asked if you would like to Export volumetric sources? enter yes. Note that Virtual Lab requires volumetric mesh data file ( <prefix>_Q_mesh.cgns) and quadrupole solution data files ( <prefix>_Q_<timestep>.cgns). The .cgns file will be stored in a similar way to that of dipole data export, in the directory specified by you in the Source Data Root File Name text entry box.